1. Field of the Invention
This invention relates to a manuscript reading apparatus utilized for a facsimile device or the like, and more particularly an improvement of a manuscript reading apparatus which reads a shading correcting reference surface formed on a manuscript guide before reading a manuscript image, stores the read image output in a memory circuit and effects an electric shading correction based on the content of the memory circuit.
2. Description of the Prior Art
Generally, in a manuscript reading apparatus for use in a facsimile device or the like, light reflected by the picture image of a manuscript illuminated by a light source is collected by a lens system to form an image on the light receiving surface of a photoelectric connecting element so as to derive out an image signal of the manuscript from the output of the photoelectric converting element.
Usually, the image signal thus obtained contains a signal distortion in which the signal level at the central portion of the manuscript is higher than that of the peripheral portion. Such signal distortion is caused by the peripheral effect of the lens, the difference in the luminous characteristics of fluorescent lamps utilized as the light source, and in the characteristic of the photoelectric converting element or the like. Such signal distortion is termed a "shading".
For correcting such shading, according to a prior art apparatus, a shading correction reference surface (for example, a white surface) of a manuscript guide disposed in the path of the manuscript is read by a photoelectric converting element through an optical system including a lens, etc. before reading the picture image of the manuscript, the read signal is stored in suitable memory means as a shading correction reference signal, then the manuscript is inserted in a manuscript reading apparatus for reading the picture image of the manuscript by the same photoelectric converting element and the read out image signal is corrected by the shading correction reference signal stored in the memory means, thereby deriving out an image signal whose shading has been corrected.
However, in the prior art manuscript reading apparatus, since the shading correction reference surface of the manuscript guide is disposed at a position contacting the manuscript, dust on the manuscript and ink particles forming images on the back face of the manuscript may deposit on the reference surface with the result that the reference signal level varies. This fails correct shading correction. For this reason, in the prior art manuscript reading apparatus, it is necessary to always maintain the shading correction reference surface in a clean state, which requires careful maintenance.